The development of short wavelength light emitting devices is of great interest in the semiconductor arts. Such short wavelength devices hold the promise of providing increased storage density for optical disks as well as full-color displays and white light sources when used in conjunction with devices that emit light at longer wavelengths. For example, blue lasers are expected to increase the storage density of optical disks by a factor of three.
One promising class of short wavelength light emitting devices is based on group III-V semiconductors, particularly group III nitride semiconductors. As used herein, the class of group III nitride semiconductors includes GaN, AlN, InN, BN, and alloys thereof, such as GaInN, AlGaN, and AlGaInN. To simplify the following discussion, “GaN semiconductors” includes GaN, and group III nitride semiconductors whose primary component is the GaN as in GaInN, AlGaN, AlGaInN.
Light emitting diodes (LEDs) and semiconductor laser diodes are fabricated on epitaxially grown layers of GaN and related alloys of semiconductor materials including an active layer that generates light by recombining holes and electrons. The active layer is sandwiched between p-type and n-type contacts to form a p-n or n-p diode structure. A p-electrode and an n-electrode are used to connect the p-contact and n-contact, respectively, to the power source used to drive the device. The overall efficiency of the device may be defined to be the light emitted to the outside per watt of drive power. To maximize the light efficiency, both the light generated per watt of drive power in the active layer and the amount of light exiting from the device in a useful direction must be considered.
GaN based LEDs and laser diodes (LDs) are fabricated by epitaxy techniques where the LED or LD layer structure is grown “p-up” on top of a relatively thick n-type GaN (or AlGaN) buffer layer which is typically grown on a sapphire or SiC substrate. The individual devices are then defined by a mesa etch through the p-n junction. The anode or p-type contact metal is applied to the top of the mesa and the specific resistance of the Schottky contact is minimized by the use of a large work-function metal such as Ni or Pd and by the use of very high p-type doping in the topmost regions of the p-type layers. For the conductive n-type SiC substrate, the cathode is a large-area ohmic contact formed by metallizing the substrate back surface. In the case of the non-conducting sapphire substrate, the cathode or n-type contact metal is applied in the field close to the mesa edge to provide a lateral current flow through the thick n-type buffer layer to the p-n junction within the mesa.
There are several drawbacks to this conventional design. First, p-type GaN is very resistive compared to other common semiconductors even when grown under optimized conditions. As a result, there is virtually no lateral current spreading in p-type GaN. To overcome this lack of current spreading, conventional p-up LEDs require a semi-transparent metal electrode that covers the p-contact to provide the lateral current spreading. This electrode reduces that amount of light that leaves the device, and hence, reduces the efficiency. In p-type AlGaN, the resistivity is even higher, increasing with increasing Al mole-fraction. Thus the p-type AlGaN layer in laser diodes used for optical wave-guiding causes further resistive heating that degrades laser performance.
Secondly, the III-V materials, in their wurtzite or hexagonal crystalline form, exhibit large spontaneous polarization charges at heterointerfaces and strong piezoelectric effects at lattice mismatched heterointerfaces, especially when grown along the [0001] crystal orientation or c-axis as is usually the case. The direction of the resulting polarization field is such that it adversely affects the optical recombination rate in a conventional p-up device.
All prior nitride-based pn junctions, which have been created for purposes of efficient light emission, have been grown by OMVPE, and include InGaN quantum wells which concentrate the electrons and holes together to achieve a large pn product and efficient radiative recombination. As a consequence of OMVPE growth kinetics, the growth along the wurtzite c-axis occurs with the so-called gallium-face exposed at the surface. This crystal orientation fixes the orientation of the polarization fields at any InGaN/GaN quantum wells in the structure. In all devices reported to date, these fields have been opposed to the built-in field of the pn junction, whose direction is fixed by the n-layer always being grown first.
Broadly, it is the object of the present invention to provide improved LEDs and semiconductor lasers based on group III-V semiconductors.
It is a further object of the present invention to provide light emitting devices with increased light output efficiency.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.